Most Baker's Yeasts presently on the market are carefully selected strains of Saccharomyces cerevisiae. They have the characters with a good yield, good keeping qualities, good dispersibility and a powerful and constant gassing power in dough.
However, in recent years, due to circumstances in the baking industry, there has been put into practice the method in which dough is frozen and stored and thawed before baking. When using a frozen dough, there is a problem as to the gassing power of baker's yeasts which have undergone such freeze-storage. When conventional baker's yeasts belonging to Saccharomyces cerevisiae are used for a frozen dough, for example, being stored at -20.degree. C. for one week or more, the yeasts will suffer freeze-injury which leads to a considerable lowering of their gassing power after thawing. Therefore, such a frozen dough fails to become a soft and full bread, even though it is subjected to fermentation and baking.
Due to such circumstances, strains belonging to Saccharomyces rosei have been developed as a freeze-resistant baker's yeast. It has, however, been shown that strains belonging to Saccharomyces rosei cannot ferment maltose and do not show sufficient gassing power under a low concentration of sugar and therefore are inferior for use as a baker's yeast in the baking of white bread, hard roll, etc., as compared to conventional baker's yeasts belonging to Saccharomyces cerevisiae.